From time to time we need to communicate with parties outside our processes. Typically, these will be customers, suppliers and regulators, but could be internal functions that we do not want to include in the model.

We represent each of these parties by a Black-Box Pool. The Black-Box Pool is empty because we do not know (and, possibly, don't care) what process the third party follows.

Message Flow

In BPMN, the representation of communication between pools is called a collaboration diagram.

Communication between our process and the third party is by means of message flow, represented by a dashed line with an unfilled arrowhead.

Message flow can only be drawn between pools.

Sequence flow, represented by a solid line and a filled arrowhead, can only be drawn within a pool.

Message flow connects the boundary of the Black-Box Pool directly with the sub-processes, activities or events within the White-Box Pool.